Depending on its application, a power transmission device may require a reverse driving preventing function. In Patent Document 1, a configuration in which a speed reduction mechanism with a starting efficiency of 40% or more is adopted as a speed reduction mechanism of a drive system, and a frictional force imparting means that will always provide resistance during operation due to friction occurring between any rotational member on the power transmission path of the speed reduction mechanism, is arranged at any position on the power transmission path, is disclosed.
This invention utilizes the difference between static friction and dynamical friction or the difference between starting efficiency and operating efficiency for realization of “reverse rotation prevention”, and has features such that the operating efficiency itself of the speed reduction mechanism of the power transmission device is high, and the starting efficiency is made low by the friction imparting means. Since the frictional force given by the friction imparting means decreases when the speed reduction mechanism starts its rotation, the power transmission device is allowed to perform the operation of enhancing its inherent efficiency once starting.
In addition, in this Japanese Unexamined Patent Application Publication No. 2004-301278, an internally meshing planetary gear mechanism having an input shaft, an eccentric body provided on the input shaft, an externally toothed gear eccentrically oscillating via the eccentric body, and an internally toothed gear with which the externally toothed gear internally meshes is taken as an example of the speed reduction mechanism with high starting efficiency.
However, since the technique disclosed in this Patent Document 1 provides “resistance caused by friction” onto the power transmission path, the degradation of efficiency (although the coefficient of dynamic friction is small) inevitably occurs accordingly. Therefore, in a device to be used while constantly moving (without repetitive starting and stopping) like, for example, a panel for solar photovoltaic power generation or a driving unit of a wind power generation system, it is considered problematic that this effect may not always be exhibited.
The enhancement of transmission efficiency of a simple drive system that makes the above “friction resistance to be given” smaller is directly connected with deterioration in a reverse driving preventing function. Recently, in a solar power generation device, the technique of irradiating a transducer with light condensed by use of a lens is also suggested in order to enhance power generation efficiency. In this device, extremely accurate positioning (tailing) of the sun is necessary. That is, a powerful reverse driving preventing function is required since neither deceleration nor acceleration is allowed in addition to reverse rotation with respect to normal driving (tailing) of the power generation panel. Additionally, even in wind power generation, it is necessary to align the direction of a nose cone of a windmill blade with the direction in which the wind blows, and to maintain the pitch of the windmill blade to a state where there is a little wind resistance in order to protect the system, for example, at the time of a typhoon. That is, the powerful reverse rotation preventing function is required even here.
Moreover, in the device according to Patent Document 1, a gear mechanism is adopted for power transmission. Therefore, since the backlash by meshing unavoidably occurs, the device is regarded as problematic, particularly in an application that requires advanced positioning accuracy. Since the backlash appears as vibration of the windmill blade, for example in the case of the wind power generation system, it is likewise but not preferable that the backlash exists.